Program Goals and Scope. Tobacco use is the foremost cause of premature death in the U.S. About 21% of adults are current smokers and smoking rates have not declined in recent years. Although available pharmacotherapies can aid in quitting smoking, quit rates vary substantially in subgroups of smokers. Thus, smoking is a significant clinical problem with a great need for research to improve treatment outcomes. The goal of the Pharmacogenetics of Nicotine Addiction Treatment (PNAT) research program is to generate the evidence base to optimize pharmacotherapeutic choices for individuals who wish to quit smoking. Building upon a strong foundation of translational pharmacogenefic (PGx) science conducted by this transdisciplinary team during the past 4 years. we propose in this competing renewal to: (a) conduct a multi-center prospective stratified PGx clinical trial to establish the predictive validity and cost-effectiveness of a genetically-informed biomarker to optimize smoking cessation treatment; (b) identify additional gene variants altering nicotine pharmacokinetics (PK), as well as pharmacodynamic (PD) gene variants influencing therapeutic response; and (c) elucidate causal mechanisms underlying associations of our PGx marker with smoking cessation.